Flap gates with relatively simple hinge structures for operations have been recently developed in forms that do not need manual operations, so that flap gates have received attention as flood-control measures. Particularly in recent years, the number of installed flap gates has increased because of the usability of flap gates and a growing awareness of disaster prevention.
In such a flap gate, a door base and the hinge structure of a substrate pivotally supporting the door base enable an operation for flapping a door. Moreover, a watertight rubber sheet for ensuring watertightness is fixed from the door base to the hinge structure substrate. If the watertight rubber sheet is separated from the pivot of a door for flapping, the watertight rubber may sag in a raised position of the door (for example, see FIG. 8 of Patent Literature 1). Thus, a structure in which the pivot of a door for flapping is disposed near (or in) a watertight rubber sheet is proposed (for example, see FIG. 1 of Patent Literature 1). As shown in FIGS. 14 and 15, a simple structure described in Patent Literature 1 has a door base 330 that is partially cylindrical and is pivotally supported by a bearing stand 302 serving as a substrate (via an oilless bush 340). With this structure, as shown in FIG. 15, even the door base 330 in a raised position does not sag the watertight rubber sheet 12. Thus, a door-side cross arm brace 14 for fixing the watertight rubber sheet 12 with countersunk bolts 16 and a substrate-side cross arm brace 15 are brought close to each other, thereby reducing the area of the watertight rubber sheet 12 exposed from the cross arm braces 14 and 15.